1. Technical Field
This disclosure relates to radar systems and methods, and, more particularly, to radar systems and methods using a modified radar signal and signal processing approach to determine relative velocity (Doppler), range and bearing of one or more objects, including simultaneous determination of relative velocity, range and bearing of multiple objects.
2. Discussion of Related Art
In a typical radar system, a transmitter transmits an electromagnetic signal from an observation point, and a receiver located at or near the observation point receives an echo signal, or reflected signal, from an object. The echo signal or reflected signal is the result of the transmitted electromagnetic signal striking the object and being reflected by the object back to the receiver. Processing of the echo or reflected signals can be performed to determine certain parameters related to the remote object, such as, for example, relative distance or radial range, relative velocity or Doppler and direction or bearing of the object with respect to the observation point.
At least two types of signals can typically be used in radar systems, namely, continuous-wave (CW) signals and chirp signals. A CW signal is generated at a single frequency. Multiple samples of the CW signal reflected from the object are taken at the receiver. The samples are typically processed to determine relative velocity of the object by detecting phase differences or phase rotations, i.e., Doppler shifts, in the reflected CW signal.
A chirp signal is an electromagnetic signal whose frequency changes with time. Generally, the frequency of an up-chirp signal increases over time, and the frequency of a down-chirp signal decreases over time. The frequency variation of a chirp signal can take many different forms. For example, the frequency of a linear frequency modulated (LFM) signal varies linearly. Other forms of frequency variations in chirp signals include exponential variations. A single pass through a chirp signal from beginning to end is commonly referred to as a “sweep.”
In addition to these latter types of chirp signals in which the frequency varies continuously according to some predetermined function, i.e., linear or exponential, chip signals can also be generated as step-chirp signals in which the frequency changes in steps. That is, a typical step-chirp signal includes multiple steps in frequency, where the frequency is constant for some predetermined time duration at each step. The step-chirp signal can also be pulsed on and off, with the pulse being on during some predetermined time period during each step of the chirp sweep. In typical step-chirp radar signal processing, object range, relative velocity and bearing information can be obtained. However, it can be difficult to eliminate ambiguities among range and velocity information, particularly in the presence of multiple objects.